Even
in its infancy - endurance management was a complicated technical subject -
but if we look back from the perspective from the ultra-complexity of today - it
was much easier to manage and understand.

Also the fastest flash SSD throughput and IOPS rates were 10x, and 100x
slower respectively than typical products today.

Nevertheless the
risks of wear-out - even in the simpler days of SSD yore - were very real. And
I've heard of many enterprise users who experienced these failures.

Since
then - as flash memory cell sizes have shrunk to deliver ever cheaper flash
capacity (more gigabytes per square inch on the chip) the raw endurance figures
in each new generation of memory have got worse.

3x to 4x better
than 2D at the same line geometries - Dave Merry
founder of FMJ Storage
told me in March 2014 - based on his early access characterization research.

Promising as that sounds - it doesn't preclude the possibility of
entirely new data
integrity sensitivities - due to anticipated 3D proximal disturbance factors
and it's too soon to be confident about new 3D-specific failure modes too.

Flash wear out still presents a challenge to
designers of high
IOPS flash
SSDs as the intrinsic effects at the cell level get worse with each new chip
generation.

That's in contrast to
RAM SSDs - where as
long as enterprise users remember to replace their batteries periodically - the
memory life is more dependent on elapsed time (classic bathtub
reliability curve)
and heat stresses rather than directly related to the number of R/W cycles.

Higher
SSD capacity, and
faster speeds come
from progressively smaller cell geometries - which we used to call shrinks.
In flash memory
small
size means less trapped charge holding the stored data values and greater
sensitivity to charge leakage, charge dumping and
disturbance effects
from the normal processes which happen around the cell vicinity during R/W,
powering up, powering
down etc.

If you're a
consumer you don't
have to worry about the internals of endurance management - because most
new SSDs are good enough (if they're used in the right applications
environment).

Exceptions still do occur, however for users in
the enterprise SSD market - where I still hear stories of users
thinking it's perfectly normal and economic to replace burned out
Intel SSDs every 6 to 12
months - instead of buying more
reliable (but more
expensive) SSDs -
from companies like STEC.

But
if you're a systems designer it's useful to know that the longevity
difference between "good enough" and the best endurance architecture
schemes can still be 2x, 3x or 100x - even when using
the same memory.

In
2011 -
new evidence started
coming in from longtitudinal flash SSD research done by
STEC that old, heavily
written MLC cells - managed by traditional endurance schemes - tend to get
slower as they get older - due to higher retry rates on reads - even though
the blocks are still reported by SMART logs as "good" - and the
writes do eventually succeed on retry.

In the same year - a paper by
InnoDisk
confirmed that whereas
SLC and MLC
memories have often had endurance populations within each chip which
were mostly much better than guaranteed (something which SSD makers had been
telling me since 2004) - the headroom / margin of goodness - in newer
types of MLC is lower than in the previous MLC generations. That's why
controllers which
used to work well with vintage MLC need something much stronger than a tweak to
deliver well behaved SSDs when co-starring with the new brat generation of
naughty flash.

That's what started the industry trends towards
designing a different type of flash management scheme -
adaptive R/W
- in which the goodness of cell blocks within the SSD are measured and
calibrated - and then different schemes of write pulse length and different
strengths of ECC
codes (including DSP - digital signal processing to remove "noise")
are applied within the same SSD.

These characteristics are
re-evaluated regularly according to error rates - but also according to the age
of the SSD and the write counts in the blocks. One of the ideas of the "age"
factor being - that using lower power write pulses at the start of SSD life
(along with stronger codes) reduces the damage done to the flash material -
which means that heavier pulses carrying more charge can be reserved for later
years of use - when the cell quality declines due to wear out effects.

This, below, is the original text of my
SSD endurance article published in
March 2007

Does
the fatal gene of "write
endurance" built into
flash SSDs prevent
their deployment in intensive
server acceleration
applications? It was certainly true as little as a few years ago (2005).
What's the risk with today's devices?

In
every other respect you can treat them in exactly the same way as a
hard drive:- same
interface, same software model. They even fit mechanically into the same
standard hard drive slots. And in many ways they are better - significantly
faster, consuming less electric power and more tolerant of ambient temperature
and vibration extremes. You mostly don't need to know about what's inside them.
They are the perfect "fit and forget" storage product.

In
the smaller form factors like 1.8"
and 2.5" - the gap
in capacity between SSDs and hard drives has disappeared. If it wasn't for the
price you'd use them - right? (The price advantage of SSDs in particular
applications is discussed in
another
article.)

What's wrong with this utopian vision?

And
why is it that even if you were offered a flash SSD accelerator for your server
absolutely FREE you might still hesitate about installing it?

When you look in more detail at flash SSDs there is just
one skinny dark stormcrow hanging around the edge of this picture which makes
you feel uneasy about a technology which in other respects is acquiring an
untarnished reputation. That's the prickly issue of write endurance.

Write
Endurance: - The number of write cycles to any block of flash is limited -
and once you've used up your quota for that block - that's it! The disk can
become unreliable.

In the early days of flash SSDs managing this was a real
headache for oems and users. The maximum number of write cycles to an address
block - the endurance - was initially small (about 10,000 write cycles in 1994,
rising to 100,000 in 1997). And the capacity of flash storage was small too.
So the write endurance limit was more than just a theoretical consideration. In
the worst case - you could destroy a flash SSD in less than a week! But in
those days the SSD was being designed in by electronics engineers who knew
exactly how the SSD was going to be used. If it helped solve the problem they
could even rewrite the software a different way to lessen the risk.

But
when you buy an SSD for use in a
notebook or
server - you don't write the software. You don't control the data. So how do you
know in advance if you're going to hit that brick wall?

This fear is
an issue which has slowed down the adoption of flash SSDs in commercial server
acceleration applications. Write endurance doesn't affect
RAM based SSDs - which have
until now dominated that part of the market - mainly due to their superior
speed. But the speed of flash SSDs has improved to the point where they could
replace RAM based SSDs
in many server acceleration slots at a much lower price - if it wasn't for the
worry about endurance.

Write endurance has been a FUD issue for
potential enterprise server users. They know it's lurking there - but who can
they trust to quantify the problem in their own language?

Server
makers didn't want users to know about SSDs (any type - period) during 2000 to
2006 - because more SSDs meant selling less servers. In the
2005
edition of the SSD Buyers Guide I wrote about the problem...

"One
disadvantage, compared to RAM SSDs is that flash has an intrinsic limit on the
total number of write cycles to a particular destination. The limit varies,
according to manufacturer but is over millions of cycles in the most durable
products. Internal controllers within the flash SSD manage this phenomenon and
can reallocate physical media transparently to prolong media life. In most
applications, high endurance flash SSDs can have a reliable operating life which
is typically 3 times as high as that of a hard drive. But I would hesitate about
installing a flash SSD as a server speedup in a university maths research
department, for example, or in other applications where the ratio of data writes
to data reads is unusually high."

In May 2006 I came to
the conclusion that my earlier doubts may need to be revised.

It was
clear from reader emails and negative comments about SSDs which I saw in other
publications that fear and doubt about the impact of write endurance was slowing
down adoption of flash SSDs in the server acceleration market. It was also clear
that most users didn't know how to interpret the kind of data being offered by
SSD oems - which was designed for an elite audience of electronics designers -
and not for managers of storage systems. So I contacted all flash SSD oems with
the idea of setting up a standard way of presenting endurance life expectancy
data - with a proposal which I called the "SSD Half Life." That dialog
met with some enthusiasm but there wasn't enough vendor support to take it
further. The SSD oems I talked to took reliability very seriously - but didn't
want their own proprietary reliability schemes and models swamped by a general
industry wide scheme.

The way that SSD oems deal with the management
of write endurance internally within their products varies but they all have the
common theme of scoring how many times a block of memory has been written to,
and then reallocating physical blocks to logical blocks dynamically and
transparently to spread the load across the whole disk. In a well designed flash
SSD you would have to write to the whole disk the endurance number of cycles to
be in danger.

Some manufacturers go a step further.
SiliconSystems
has a patented algorithm which delivers a lifetime which it claims is better
than simplistic wear levelling. Another manufacturer
Adtron actually has a
percentage of spare flash blocks in the SSD - which are invisible to the host
interface and don't show up as spare storage. But internally - when blocks get
close to the limit - the data is transparently switched over to the spare parts
of the disk to give an additional breathing space.

The precise numbers
are a proprietary secret but are based on analyzing the software from real
customers' SSD applications over many years. OEMs, like these, which target
high reliability applications, are also more picky about which flash chips they
use, and qualify them according to the results they see from testing.

the
Flash SSD Application from Hell* - the Rogue Data Recorder

In
most real-life applications the computer does a lot more reads from disk than
writes - and the duty cycle (that's the percentage of time that the disk is
being accessed at all) is low. But to estimate whether you should be worried
about write endurance with today's SSD technology I've chosen a worst case
example - the Rogue Data Recorder.

Real
hard disk based data
recorders from companies like
Conduant can record
data continuously in an endless loop. They are useful for a bunch of
applications such as capturing pre-trigger data in seismic events, capturing
unpredictable data for modelling and bugging phone calls. I managed a company
in the mid 80s which pushed storage technology to its limits to get wire speed
continuous recording onto disk and massive memory systems with inbuilt real-time
trigger processors, embedded workstations and array processors for various
types of industries and agencies. That was a good education for my day job now
of cutting and pasting.

Most of you wouldn't set out to design a
real-time data recorder - and if you are doing that - this article isn't going
to tell you anything you don't already know. But by looking at the worst thing
which could happen and estimating a confidence boundary from that - it can tell
you how much you need to worry.

The nightmare scenario for your new
server acceleration flash SSD is that a piece of buggy software
written by the maths department in the university or the analytics people in
your marketing department is launched on a Friday afternoon just before a
holiday weekend - and behaves like a data recorder continuously writing at
maximum speed to your disk - and goes unnoticed.

How long have you
got before the disk is trashed?

For this illustrative
calculation I'm going to pick the following parameters:-

Configuration:-

a single flash SSD. (Using more
disks in an array could increase the operating life.)

Write
endurance rating:-

2 million cycles. (The typical
range today for flash SSDs is from 1 to 5 million. The technology trend has been
for this to get better.

When this article was published, in March
2007, many readers pointed out the apparent discrepancy between the endurance
ratings quoted by most flash chipmakers and those quoted by high-reliability
SSD makers - using the same chips.

In many emails I explained that
such endurance ratings could be sample tested and batches selected or rejected
from devices which were nominally guaranteed for only 100,000 cycles.

In such filtered batches typically 3% of blocks in a flash SSD might only last
100,000 cycles - but over 90% would last 1 million cycles. The difference was
managed internally by the controller using a combination of
over-provisioning and
bad block
management.

Even if you don't do
incoming inspection
and testing / rejection of flash chips over 90% of memory in large arrays can
have endurance which is 5x
better than the minimum quoted figure.

Since publishing this article,
many oems - including
Micron
- have found the market demand big enough to offer "high endurance"
flash as standard products.)

That's a meaningless number - which needs to be divided by
seconds in an hour, hours in a day etc etc to give...

The end result
is 51 years!

But you can see how just a handful of years ago -
when write endurance was 20x less than it is today - and disk capacities were
smaller.

For real-life applications refinements are needed to
the model which take into account the ratio and interaction of write block size,
cache operation and internal flash block size. I've assumed perfect cache
operation - and sequential writes - because otherwise you don't get the maximum
write speed. Conversely if you aren't writing at the maximum speed - then the
disk will last longer. Other factors which would tend to make the disk last
longer are that in most commercial server applications such as databases - the
ratio of reads to writes is higher than 5 to 1. And as there is no wear-out or
endurance limit on read operations - the implication is to increase the
operating life by the read to write ratio.

As a sanity check - I found
some data from
Mtron (one of the few
SSD oems who do quote endurance in a way that non specialists can understand).
In the data sheet for their
32G product - which incidentally has 5 million cycles write endurance - they
quote the write endurance for the disk as "greater than 85 years assuming
100G / day erase/write cycles" - which involves overwriting the disk 3
times a day.

How to interpret these numbers?

With
current technologies write endurance is not a factor you should be worrying
about when deploying flash SSDs for server acceleration applications - even
in a university or other analytics intensive environment.

The calculation above gives the worst case (shortest)
operating life based on stuffing data into a single disk at the fastest
possible speed. Having a faster interface coming into the a
box stuffed with SSDs
doesn't make the life shorter - because the data can only be striped to any
individual disk at the limiting rate for that disk.

Au contraire:- not
only can an SSD RAID array offer a multiple of a single SSD's throughput,
and IOPs, just as with hard disks but depending on the array configuration the
operating life can be multiplied as well - because not all the disks
will operate at 100% duty cycle. That means that MTBF and not write endurance
will be the limiting factors. And although oem published
MTBF data for hard
disks has been discredited recently - the MTBF data for flash SSDs has been
verified for over a decade in more discriminating applications in high
reliability embedded systems.

I've been waiting years for storage oems
to start marketing flash SSD based storage arrays - as alternatives to RAM based
systems. What's held that market back has been the looming shadow of write
endurance. That myth - that flash SSDs wear out - now belongs to the past.

...Later:-
in May 2008 - in an exclusive
interview with STORAGEsearch.com -
AMCC 3ware confirmed it is
working with leading
SSD oems to develop
products which will support the unique needs of the
flash SSD RAID
market. * clarifying why the Rogue Data Recorder is the Worst Case
Application

I didn't need to explain this choice to those
who design SSDs, but it's clear from some comments I've seen that some readers
who don't have an electronics / semiconductor education or don't know enough
about SSD internals have queried this choice.

Why, for example, does
the data recorder example stress a flash SSD more than say continuously writing
to the same sector?

The answer is that the data recorder - by writing
to successively sectors - makes the best use of the inbuilt block erase/write
circuits and the external (to the flash memory - but still internal to the SSD)
buffer / cache. In fact it's the only way you can get anywhere close to the
headline spec data write throughput and
write IOPS.

This is because you are statistically more likely to find that
writing to different address blocks finds blocks that are ready to write.

If
you write a program which keeps rewriting data to exactly the same address
sector - all successive sector writes are delayed until the current erase /
write cycle for that part of the flash is complete. So it actually runs at the
slowest possible write speed.

If you were patient enough to
try writing a million or so times to the same logical sector - then at some
point the internal wear leveling processor would have transparently assigned it
to a different physical address in flash by then. This is invisible to you. You
think you're still writing to the same memory - but you're not. It's only the
logical address that stays the same. In fact you are stuffing data throughout
the whole physical flash disk - while operating at the slowest possible write
speed.

It will take orders of magnitude longer wearing out the memory
in this way than in the rogue data recorder example. That's because writing to
flash is not the same as
writing to RAM, and also
because writing to a flash SSD sector is not the same as writing to a block of
dumb flash memory. There are many layers of virtualization between you and the
raw memory in an SSD. If you write to a dumb flash memory chip successively to
the same location - then you can see a bad result quite quickly. But
comparing dumb flash storage to intelligent flash SSDs is like comparing the
hiss on a 33 RPM vinyl music album to that on a CD. They are quite different
products - even though they can both play same music....Later:- Clarifying Flash Endurance Specifications

I've
added this footnote in response to some reader emails which asked about the
variation in flash endurance specs quoted by different flash SSD oems.

Like
any semiconductor related spec (such as memory speed, or analog offset voltage
in an op-amp, or failed memory blocks in a high density RAM chip) - there's a
spread of performance which depends on the process and may vary over time in
the same wafer fab, or at the same time when chips are made in different fabs
within the same company.

A spec such as 100k or 1 million or 10 million erase-write cycles -
is a business decision made according to market conditions - which gives
generic semiconductor buyers a confidence level that if they buy 1 million
chips - then the reject rate - of those that will fail due to process
tolerances - will be acceptably low. The shape of the distribution curve may
not actually be gaussian - but there is a distribution curve in there which is
implied by the published specs.

Due to process variations between oems
(some designs will be automatically shrunk from old designs, other layout
geometries may be recompiled or optimized for that particular process point)
there will be vast differences between the endurance from different
chipmakers.

As the generic semiconductor flash market doesn't place a premium on
this spec - the "datasheet" published standard number will gradually
improve at a slow pace (every 2-3 years) even if some oems are making chips
today which are 10x better.

If I was designing a
high reliability
flash SSD - I would want to get into the process details - qualify devices and
order them to my own spec. Currently SSD volumes are too low - to give much
buying power with flash chipmakers. Therefore few SSD oems are able to buy
flash chips qualified to their own specs. (This is done by batch testing
samples and by negotiation with the fab where the chips are made.) Some SSD oems
make their own flash chips - and while this gives them more control over the end
to end process - it does not necessarily mean that they start with the best
chips.

In the
1970s - endurance in EAROMs was about 1,000 write / erase cycles per
cell.EAROMs were remote ancestors of flash memoryfrom a systems
designer's point of view but were different inside re materials
technology.

Currently there is no type
of mainstream nand flash which isn't being used in some type of enterprise SSD
systems.

And if you hear some enterprise vendors say - that their array is
better because it uses so called enterprise MLC (eMLC) it really means that
they don't know how to manage the flash with their own IP and have passed the
buck to their memory suppliers and to their customers (who have to pay more).

In December 2012 -
Macronix
described an experimental design of flash cells which they said could be
used to refresh endurance to over
100 million cycles.

.

endurance
issues in flash SSD operating life

Editor:- July 1, 2011 - In my
2007 article (lower down on the left of this page) I showed readers why flash
SSD operating life could be much greater than predicted by extrapolating flash
chip wear out - due to a blend of architecture and process choices
implemented in well designed SSDs.

Before
2007 - the
industry wisdom had been that users should not use flash SSDs in
enterprise apps.
(And in those days - we were talking about SLC flash.)

But wear-out was
then (and remains today) a reliability risk in badly designed SSDs! And I
still get a lot of reader questions about it.

For those who just need a
short summary of the issues - here it is.

Wear leveling is more effective in some SSD designs than in
others. The effectiveness of patented active wear leveling algorithms can
vary by
more than 3 to 1.

In some good controllers the gigabytes written to
the flash are actually
less than the gigabytes written to the SSD from the host. Conversely in
naiively designed controllers the flash media to host write ratio can be
20x higher - resulting in very short life in enterprise apps. This is
not just a theoretical issue. Real enterprise customers
have
encountered system failures when consumer SSDs were installed into heavy
duty arrays.

All flash chips
are not the same - even from the same fab (chip factory).

All
flash cells do not have the same endurance even within the same chip.

High
reliability oems have been
sample
testing and rejecting flash chips since 2004 to my knowledge - to select
flash chips which are 5x or 10x better than the minimum quoted figures for
endurance by chipmakers. So called "enterprise grade flash" is simply
a marketing label to industrialize a process which had already been used
in the industry below the visibility radar of most storage users.

Over-provisioning extends SSD life - because all cells in a chip do
not have the same endurance.

There's a distribution curve of
endurance within chip blocks which is a proprietary secret which can be
characterized by the SSD controller designer for the chips they support. Most
blocks are significantly better than the floor level in the same memory chip.

On the other hand - if an SSD company installs the wrong flash chips
(maybe due to market supply problems) - the SSDs may work in the short term -
but fail early. This is because the mortality rate of cells may be outside
the bands and coping strategies built in to the controller for the type of
chips it was designed to work with. But the same chips could still be usable in
an entirely different set up of SSD controller.

It's like putting
the wrong tires on your car. Everything's OK on the daily commute. You see
the difference when you hit snow or drive around a bend at 120 mph.

It's this combination of factors which can make a difference (up to 2 orders of
magnitude) between SSD life - even when using flash made by the very same chip
supplier

Through its QA processes the
company has acquired real-world failure data for several generations of
flash memory and used
this to model and characterize the failure modes which occur in high IOPs
SSDs.
...click to read
more

"In
many applications that use a standard file system, the directory updates are a
major concern for using up the available flash life.

Even though
recording applications are inherently heavy on writing, the optimization of the
directory structure actually minimizes the number of times a specific location
is written thereby extending the life of the flash media.

This is on top of any wear leveling algorthms provide by the SSD
manufacturer. Conduant systems can even use Compact Flash in some recording
applications."

.

DRAM Memory Doesn't Wear
Out

In the
2004
edition of the SSD Buyer's Guide I warned users in no uncertain terms that
they couldn't use flash SSDs in enterprise server apps - because of the low
endurance of products available at that time.

The banner ad, below,
was run here in February 2004, by a company called
Computer Expertise Group
(no longer in business). CEG sold refurbished
RAM SSDs - and the
banner includes the tagline - "DRAM memory doesn't wear out".When this ad was run, that statement wasn't an allusion to flash
wear-out (because no one in their right minds would have considered flash for
server apps at that time). Instead it suggested that whereas a refurbished
hard drive array won't last as long as a new one, a refurbished RAM SSD could
be just as good as new - and cheaper (assuming you also replaced the batteries.)

This article (which has become unexpectedly
popular) will help you understand why some SSDs which work perfectly well in
one type of application might fail in others... even when the changes in the
operational environment appear to be negligible.